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Seeking advice on furnace & AC replacement

I have a 20 year old CM 150,000 btu furnace (all 4 furnace pros I've had out have said it was way oversized) and a 17 year old 3 ton Miller AC in my 45 year old ranch with 1700 sq. ft. living space and 1500 sq. ft. basement in a Detroit suburb. I've been educating myself, talked to 4 HVAC pros and would rather spend more $$ up front to lower annual heating/cooling costs as I expect natural gas to keep rising. The furnace I think I'm going with is the Amana AMV90704CX, which has an AFUE of 95.5% and a 14 or 16 SEER AC, such as the ASX140361A. The company I think I'm going with is small; owner, secretary, 4 techs and they've been in business almost 30 years. He seems honest and knows his stuff and a couple of the techs have been with him 10 and 20 years. I have asked a lot of questions and have heard satisfactory answers for the most part but I have a couple concerns and would like some advice. I've read gradual curves are "required" on the pvc intake and exhaust but this guy said he uses sharp corners but uses 3 inch pvc where 2 inch is called for, as I believe my probably new furnace does. The other concern is he doesn't "strictly" use the Manual-J method of calculating the required sizes. He said he looks at home age, design, insulation, windows and uses experience. None of the 4 pros I've talked use seem to use the full Manual-J method but some modified version. Should these two concerns bother me? What other questions could I ask to insure I'm getting a good installer as I've read brand is not nearly as important as the installation. Thank you so much for such a great resource!
Jeff

The calculator comes out to 2.8 tons for your house size.. maybe thats why one guy said 2.5 or 3 ton

I am in the same boat.. kinda.. cuz my house is 1200 square feet..... the calculator says i need 2.2 tons.. i've had quotes for 2 ton and 2.5 ton. I am not sure if its better to go bigger or smaller than you need

The calculator comes out to 2.8 tons for your house size.. maybe thats why one guy said 2.5 or 3 ton

I am in the same boat.. kinda.. cuz my house is 1200 square feet..... the calculator says i need 2.2 tons.. i've had quotes for 2 ton and 2.5 ton. I am not sure if its better to go bigger or smaller than you need

That thing is just a guess.

For my place(1650 sq ft) it says 3 tons.
I have 2.5 tons, and thats over sized for my place by 9,000 BTUs.

There is no quick 1 minute calculation that will tell you what size you need.

Thanks for the feedback everyone. Of the 4 pros I've had out here, 1 never got back to me - hard to imagine why in what is supposed to be a bad economy. The other 3 all seem competent and I'm kind of down to two based on recommended models. Of those two the prices are significantly different. They seem equally competent so I'm wondering does the lower guy not know what he's doing or is the higher guy gouging? At this point I'm inclined to believe it's the latter. Any suggestions on questions to ask to insure they really know their stuff?

Look at the proposals/estimates.
Are they both doning the same things.

Maybe the higher priced guy is doing more. Maybe not.

Higher guy might be using sheet metal, and the lower guy might be using duct board transitions.
Are any of them testing performance.
The higher guy, may send his men to more training then the other. ( You do want trained techs working on your equipment don't you )
If they gave you model numbers. Post them here.
One might be using a higher line of equipment then the other.